I don't think that large nymphs (eg stonefly, dobsonfly), cased caddis, crayfish are all that soft. Soft materials may give the advantage of movement but doubt that beadheads, stiff feathers, plastic shellbacks, etc make a huge difference. Also, fish are used to sucking in other materials along with the "food". The most important thing (IMHO) is whether the materials increase the likelihood of the fish actually putting the fly in it's mouth in the first place.
Keith